/\A   111 
  [.] 255 
  [.] ( \d | [1234]\d | 5[012345] )
  [.] ( \d | [1234]\d | 5[012345] )
\z/x;
[download]

...should work ( with /x modifier used to enhance readability ), but if you're not limited to a regexp, a cleaner ( and more easily reusable!) solution would be something like...

return 1 if 
  /\A 111 [.] 255 [.] (\d+) [.] (\d+) \z/x
  && $1 <= 55
  && $2 <= 55;
[download]

    --k.

Update: Fixed typo; thanks CS!